1. Field of the Invention
The present invention relates generally to the field of electronic filters. In particular, the present invention relates to an adaptive balancing network apparatus that provides improved echo cancellation of a transhybrid response.
2. Description of Related Art
Using the existing telephone system as a communication channel for the additional services involves more than just connecting a machine to a phone line. For example, the existing phone system was only designed to transmit analog signals with a bandwidth of about 300-3500 hertz, which is sufficient for voice communication, but not optimum for the additional services which generally transmit data having significantly higher frequency components. The result is that the signals of the additional services distort as they propagate.
Aggravating the problem of using the present system is the fact that only a single twisted pair of wires interconnect a subscriber (user) with a central office. This means that the subscriber's transmitter (mouthpiece) and receiver (earpiece) share the same two wires. While this is a low cost method of connecting subscribers to a central office, because full duplex (simultaneous two-way) operation is desired cancellation of a subscriber's transmission from his own receiver is required.
The function of a receiver is to detect pulses being sent from the far end of the communication channel. Since the communication channel is only a two wire cable, transmit and receive pulses can be contemporaneous on the channel causing an echoing effect when the two interact. The echoing effect can be removed by an echo canceller using a replication or a portion thereof of the transmitted pulse and subtracting it from the received pulse, such as described in the co-pending and commonly assigned U.S. patent applications Ser. No. 07/507,593, entitled "IMPROVED TRANSVERSAL FILTER ECHO CANCELLER," filed Apr. 10, 1990, by Kenneth G. Buttle, and Ser. No. 07/507,595, entitled "NONLINEAR ECHO CANCELLER", filed Apr. 10, 1990, by Kenneth G. Buttle et. al., both of which applications are incorporated by reference herein.
This is understood by considering that if a first and second subscriber both transmit at the same time, the second subscriber's signal attenuates as it travels toward the first subscriber. If the first subscriber does not cancel out of his receiver his own transmission he will hear only his own transmission and not the desired, attenuated second subscriber's signal.
Full duplex operation on two wires therefore requires that a transmitted signal must be reduced sufficiently from the transmitter's own receiver to allow reception of an incoming signal. While readily accomplishable with voice communication, it is much harder to do with the additional services because those services are more sensitive to incompletely cancelled signals.
Exacerbating the problem of increased sensitivity to incompletely cancelled signals are the numerous wire taps, wire gauge changes, and switching networks which cause signal "reflections" in the present phone system. These reflections can be picked up by the transmitter's receiver and, if not handled properly, could be mistaken for a signal from another subscriber.
Even further complicating the use of the present phone system as a communication medium is that the present phone lines have widely variable transmission line characteristics. This creates a problem because any mismatch between the telephone line and the service using the phone line causes an "incident" signal that is reflected into the receiver.
The effects of insufficient echo cancellation, line taps, mismatches, and high frequency are more serious when using the additional services than with just voice communications. To assist in reducing these and other problems, the Accredited Standards Committee on Telecommunications, T1, submitted to the American National Standards Institute a standard for integrated services digital networking, ANSI T.601-1988, entitled "Integrated Services Digital Network (ISDN)--Basic Access Interface for Use on Metallic Loops for Application on the Network Side of the NT (Layer 1 Specification)." This document describes a minimal set of requirements and protocols for satisfactory communication between subscribers when using the additional services with full duplex operation on the existing single twisted wire pair phone line.
The ANSI T1.601-1988 specification defines "basic access" as a standardized combination of access channels that constitute the access arrangement for the majority of ISDN users. Specifically it includes any of the following combinations of access channels:
a) one D-Channel PA1 b) one B-Channel PA1 c) two B-Channels & one D-Channel
where a B-channel is a 64 kilobits per second (kps) channel that carries customer information, such as voice calls, circuit switch data, or packet switch data; a D-channel is an access channel carrying control or signaling information and optionally packetized information and telemetry. The D-channel has a capacity of 16 kps. Accordingly, the data rate transfer sum of two B-channels and one D-channel is equal to 144 kps. Problems arise on these high speed channels since data is transceived over a telephone voice communication channel having a narrow bandwidth and line insertion losses of 40 to 50 dB.
As previously indicated, for acceptable full duplex operation when using the existing single twisted pair phone lines with the additional services, the transmitted information must be more fully cancelled despite the increased reflections and the incident signal. By adding together both the reflections and the incident signal, one obtains the "transhybrid response."
A signal transmitted on a phone line may become attenuated by as much as 40 dB (100 times), making a 5/6 volt transmit signal only about 0.0083 volts at the receiver. Since an echo may be almost as large as a transmit signal, or up to about 2.5 volts, a receiver may be required to detect a 0.0083 volt signal riding on a 2.5 volt echo. Reduction of the echo amplitude to an acceptable amount is the job of the echo canceller.
While the echo canceller must reduce the echo to an acceptable amount (roughly a 67 dB reduction), the actual transhybrid response is unknown until the lines connecting the subscribers are established, something that does not occur until a call is answered. Therefore, acceptable methods of echo cancelling must be adaptive, i.e., they must adjust to the line conditions existing at the time of the call.
Usually, a total echo canceller consists of a balancing network and an adaptive echo canceller. Typically, the adaptive echo canceller is a transversal filter implemented with digital circuits which eliminates the transhybrid response by adjusting its output to cancel the response. The balancing network, which is usually a simple fixed divider, is typically implemented with analog circuits.
Therefore, one way to implement a total echo canceller is to convert analog signals output from the balancing network to digital signals so that the transversal filter can cancel a residual echo in a digital domain. Another way to implement a total echo canceller is to convert digital signals output from the transversal filter to analog signals so that the balancing network can cancel a residual echo in an analog domain. In either case, the key issue is the accuracy requirement of the A/D or D/A convertors, because the achievable transhybrid cancellation with a balancing network implemented by a simple divider is only 6 dB due to gage differences, bridged-tap connections, etc. The accuracy requirement for the A/D or D/A convertors is 13 bits or more, if the balancing network is implemented by a simple fixed divider. However, such accuracy in A/D and D/A convertors cannot be easily achieved in a cost-effective manner using current IC fabrication techniques. The present invention eliminates this problem using an adaptive balancing network which amplifies the signal before the A/D or D/A conversion.